1. Field of the Invention
The present invention relates to planar parallel mechanisms (i.e., planar parallel manipulator) and, more particularly, to a planar parallel mechanism having a platform movable in a plane along three degrees of freedom (x, y, θ).
2. Background Art
Different precision table units have been provided for displacing and positioning a moving platform in a plane. Precision table units are found in various uses, for instance, for the precise positioning of specimens, as in a microscope stage, or for precise fiber alignment, as in an alignment stage.
Most such table units are based on a serial architecture, but some have parallel architectures. In a typical table unit based on a serial mechanism, a rotary stage is mounted on top of a single-axis translation stage, which in turn is mounted on another single-axis translation stage. Such a stack-up configuration has three degrees of freedom, being two translations along X and Y axes and a rotation about Z axis.
Such a serial mechanism has the advantage of great simplicity in its control of motion. If the table unit is to be rotated, only the rotary stage is set in motion. If the table unit is to be displaced along the X axis, only the translation stage whose axis is parallel to the X axis is put in motion. Such a property is called decoupling. The relationship between the inputs, which are the positions of the actuators, and the output, which is the position and orientation of the moving platform, of such a mechanism is decoupled.
Decoupling is advantageous because of the simplicity of motion control, and also because it ensures that the table unit has a constant output resolution at any position and orientation. A major drawback is that serial mechanisms are self-supportive, whereby a ground-based actuator must support the weight of other actuators, in addition to the load. Therefore, for some movements, some of the relatively heavy actuators are displaced together with the moving platform, and therefore these table units are relatively sluggish.
Parallel mechanisms have a plurality of supporting members called legs, each separated from one another (i.e., in parallel). Consequently, a load supported by the moving platform is split into smaller loads for each leg. Parallel mechanisms are also advantageous in allowing the actuators of the legs to be base-mounted. Consequently, for a same object to be moved, parallel mechanisms involve substantially smaller loads set in motion than would require a serial mechanism.
One major disadvantage of most parallel mechanisms is that the relationship between the input motion from the actuators and the output motion of the moving platform is highly coupled and nonlinear. An input-output relationship is said to be coupled, in the case of a three-degree-of-freedom planar parallel mechanism, when for a motion of the moving platform along X or Y axis, or about Z axis, more than one actuator are to be driven at different rates. An input-output relationship is said to be nonlinear when, for a given position and orientation of the moving platform, there exist several possibilities for the positions of the actuators, or when for given positions of the actuators, there exist several possibilities for the location of the moving platform.
Various so-called planar parallel mechanisms whose moving platforms undergo a planar three-degree-of-freedom controllable motion have been provided in prior art and some have found application in industry.
U.S. Pat. No. 6,622,586, issued Sep. 23, 2003 to Scheidegger et al., entitled “Positioning Device,” discloses a three-degree-of-freedom planar parallel mechanism whose moving platform has three linear guides arranged in a symmetrical Y-shape and whose actuators drive three pins along a common circular path, each pin restrained to move along one of the three guides of the moving platform. This design has the advantages of an unlimited rotational capability and relatively simple input-output relationship. However, this relationship is highly coupled.
The publication “PRP Planar Parallel Mechanism in Configurations Improving Displacement Resolution,” by Stéphane Ronchi et al. (proceedings of the First International Conference on Positioning Technology, Hamamatsu, Japan, Jun. 9-11, 2004), discloses a similar three-degree-of-freedom planar parallel mechanism for precision positioning whose moving platform has three linear guides arranged in a symmetrical Y-shape and whose linear actuators drive three pins along the sides of an equilateral triangle, each pin restrained to move along one of the three guides of the moving platform. This design, too, has the advantage of a relatively simple input-output relationship. However, this relationship is still highly coupled.
U.S. Pat. No. 5,163,651, issued on Nov. 17, 1992 to Matsumoto, entitled “Movable Table,” discloses a positioning device based on a three-degree-of-freedom planar parallel mechanism. The moving platform of this mechanism has three linear guides arranged in a T-shape, for which base-mounted linear actuators drive three pins along the sides of a Π-shape, each pin restrained to move along one of the three guides of the moving platform. This mechanism has a relatively simple input-output relationship, which is, however, completely coupled.
The publication “Singularity Analysis of 3-DOF Planar Parallel Mechanisms via Screw Theory,” by Ilian A. Bonev et al. (Journal of Mechanical Design, Vol. 125, September 2003), presents kinematic analyses of all general three-degree-of-freedom planar parallel mechanisms with three identical legs. In this reference, it is shown that most planar parallel architectures have complex input-output relationships that are flawed by singularities near which the output precision is significantly deteriorated. Conditions for simplifying some designs are given but no new specific examples are shown.
The publication “Fully-Isotropic Over-Constrained Planar Parallel Manipulators,” by Grigore Gogu [proceedings of 2004 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2004), Sendai, Japan], discloses all possible designs of theoretically perfect planar parallel mechanisms with three legs. The kinematic model of these parallel mechanisms is the same as or nearly the same as that of a serial mechanism. However, these parallel mechanisms are not suitable for precision positioning devices because they have too many joints and offer little support to the moving platform.
U.S. Pat. No. 6,196,138, issued on Mar. 6, 2001 to Sakai et al., entitled “Movable Table Unit,” provides a three-degree-of-freedom planar parallel mechanism that basically consists of a translation stage on which a two-degree-of-freedom parallel mechanism is mounted. In the latter, three points of a moving platform are constrained to move along three linear guides that are normal to the axis of the translation stage. Two of the points are displaced along the linear guides through actuators. The proposed mechanism therefore has a simple input-output relationship. The linear stage controls directly the motion along an X axis, the other two linear actuators control directly the motion along a Y axis when moved in the same direction, while when these two linear actuators are moved in opposite directions, they control the rotation about a Z axis. However, the mechanism of U.S. Pat. No. 6,196,138 is not truly parallel and has the same disadvantage as a serial mechanism, in that the last two actuators are not base-mounted.
U.S. Pat. No. 6,635,887, issued on Oct. 21, 2003 to Kwan et al., entitled “Positioning System for Use in Lithographic Apparatus,” discloses a three-degree-of-freedom planar parallel mechanism that consists of a translational stage which is transversely mounted onto two other parallel translational stages through flexure joints. When the two base-mounted actuators move in the same direction and at the same rates, the moving platform uniquely translates along an axis parallel to the directions of the two base-mounted actuators. When the two base-mounted actuators move in opposite directions and at some specially prescribed rates, the moving platform uniquely rotates. Finally, when, the third actuator is displaced, the moving platform is displaced solely along the direction of the third actuator. This mechanism has the disadvantage of not having all of its actuators mounted on the base. Furthermore, the range of rotations of this mechanism is severely limited due to the flexure joints.
U.S. Pat. No. 5,523,941, issued on Jun. 4, 1996 to Burton et al., entitled “X-Y-Theta Positioning Mechanism,” discloses a three-degree-of-freedom planar parallel mechanism that is driven by three base-mounted linear actuators that push the moving platform through rollers. While the input-output relationship may be approximated as simple for small rotations of the moving platform, it becomes extremely complex for larger rotations.